In a conventional FM demodulation system that uses low IF (intermediate frequency) signals with frequencies of several hundred kHz and various structures such as an analog IF filter or an analog-to-digital converter (ADC) and a digital IF filter or a combination of an analog IF filter and a digital IF filter have been used to reject the interference signals contained in received signals.
In a structure with an analog IF filter, the properties of the analog IF filter can be affected by temperature and element, variations. In addition, it is difficult to reject the interference signals completely with an analog IF filter due to its mild filtering properties. In addition, it is necessary to make a complex internal structure with an analog IF filter to increase its performance in rejecting the interference signals, which increases the size of a circuit.
If the structure uses a digital IF filter, then the intended filtering properties can be obtained by using a digital process; however, the use of an ADC because it increases the consumption of electric current and typically requires a very high signal-to-noise (S/N) ratio.
In a structure with a combination of an analog IF filter and a digital IF filter, it is possible to pass the signal through the ADC and digital IF filter after removing some of the interference signal using an analog IF filter, which limits the ADC spec required. However, the size of the circuit is still large due to the need to use two types of filters and an ADC, thereby making the overall structure complex.